Grassroots Motorsports
Issue link: http://grassrootsmotorsports.epubxp.com/i/501675
Grassroots Motorsports 154 UPPER CONTROL ARM LOWER CONTROL ARM PROJECT CAR: NISSAN 350Z Springs are the mechanical devices that control suspension articulation in hopes of keeping the tires on the ground. A tire that's not on the ground can't pr and traction equals trophies. It would stand to reason, then, that using the softest springs possible will be best for handling. Soft springs mor react to irregularities and help the ound. Well, that's true to a point. The r e are a million other variables involved in a car's handling: roll angle, camber change due to suspension motion–even something as small as an individual driver's grip on the wheel can mea- fect a car's performance. be that running the softest springs you can get away with will be best for handling. So that brings us to the Z. When we start looking at springs, we need to remember to think in terms of wheel measured rate of the spring. That's a unique motion ratio–the amount of leverage on the spring gained or lost fguring the difference between two measurements: the distance between the inner suspension pivot point and the center of the contact patch of the tire, and the distance between that same inner suspension pivot and the center- line of the spring. When a MacPherson strut- equipped car has the spring placed ratios ar With the Z, however, the spring is positioned quite a bit inboard from the contact patch, resulting in a rather aggressive 0.688 front motion ratio and a 0.649 rear motion ratio. To calculate the actual spring rate at the wheel–known as wheel rate–we e of the motion ratio. The next thing we have to look at is the suspension fr . This is basi- ement of the speed and force with which a suspension reacts to inputs from the driver and the road. Fr essed in hertz. For comparison purposes, most street cars will be somewhere in the 1.1-1.5-Hz range. Sportier cars will be in the high 1s, while dedicated production-based track or autocross cars exceed 2 Hz. Slick-tired tube-framed or GT cars can exceed 3 or even 4 Hz, while formula cars can run as high as 4-5 Hz. Extreme-downforce cars can run even higher, but aero forces add a whole new level of complication to SPRING INTO KNOWLEDGE: THE MATH BEHIND SUSPENSION SPRINGS These formulas should get you where you need to go to begin choosing spring rates based on desired suspension frequency. Remember that whatever your frequency is, you must have shocks capable of damping that frequency. CENTER OF CONTACT PATCH INNER CONTROL ARM PIVOT MOTION RATIO = A/B A B SPRING CENTERLINE MOTION RATIO WHEEL RATE = SPRING RATE * (MOTION RATIO) 2 SUSPENSION FREQUENCY = Pi * √ (wheel rate/sprung weight at that corner)